Molding of materials is well known and is especially useful in forming parts of plastic materials. Compression and injection molding are two of the most used plastic molding techniques for molding both “pure” plastics and reinforced plastics. Impregnating reinforcing fibers with a plastic significantly enhances the mechanical strength of the molded parts. It is desirable that the reinforcing fibers are evenly distributed throughout a part. In parts having complex shapes and variations between thick and thin sections, finished parts often lack reinforcing fibers in the thin sections. Upon entry to a thin section, reinforcing fibers collect and form a bridge across the opening into the thin section resulting in reduced fiber density in the thin section. This is, in part, due to the fact that the constant hydraulic pressure throughout the charge results in a small force when multiplied by an area of an entrance into a section of small cross sectional area. Another factor is that when a reinforcing fiber “hangs up”, fluid will flow around it and reduce the force on the fiber. This is particularly apparent in structures like bolts that have a central section of large cross section and protruding sections (threads) of much smaller cross section.
Another structure exhibiting a large cross section with protruding sections of smaller cross section is a sabot. A sabot is placed on the circumference of a rod shaped artillery projectile when the projectile is of smaller diameter than the gun barrel. With the sabot and projectile filling the entire cross section of a gun barrel, there is more force applied to the projectile and it is guided straight through the gun barrel. Upon exiting the gun barrel, the sabot breaks free of the projectile.
It is imperative that the sabot remain with the projectile until it leaves the gun barrel. This may be achieved by forming external parallel grooves and ribs on the projectile and by forming mating internal parallel grooves and ribs on the sabot. These parallel grooves and ribs are referred to as buttress threads. Because of the high forces experienced by the projectile/sabot assembly within the gun barrel, the buttress threads must have high strength. Sabots are presently made of aluminum and are required to have shear strength sufficient to withstand gun chamber pressures of 60,000 psi. Because it is advantageous that the sabot be lightweight and, inexpensive, it is desirable to use reinforced plastic to make the sabot. Conventional plastic forming techniques, compression molding, or injection molding leave the buttress threads without reinforcing fibers and without sufficient strength to reliably retain the sabot with the projectile. According to unpublished data of the US Army Armament Research Development & Engineering Center, Picatinny Arsenal, sabots formed by conventional molding techniques have exhibited thread shear strengths from about 6,000 psi to about 12,000 psi with an average of about 8,000 psi for a sample of 9 tests, and failed under gun chamber pressures between from about 10,000 psi to about 13,000 psi.
In either compression or injection molding, the charge has a very high viscosity. Hence, unlike water that cannot support a force in a given direction, the charge tends to flow in the direction of the applied pressure and is resistant to flow laterally into interstices. It is this viscous flow resistance that inhibits movement of reinforcing fibers into interstices. Thus, standard molding practice of deforming the entire charge results in under-reinforced thin sections or interstices.